Who never noticed this icon which appears at the top every document window, to the left of the window title, that we call “proxy” icon. Although these icons seem to be added almost as a decoration to the top of the windows, they are filled with somewhat hidden features that are very useful, particularly for drag and drop. Here is an example of proxy icon, on the left of the document’s (in TextEdit):
We will see in this text the features offered by the proxy icons located in the windows’ title-bar. It is recommended to know the bases of drag and drop with files before reading this text.